1. Field of the Invention
This invention relates to a process for treating a silica organosol to make the silica more organophilic by reacting the silica with a trialkoxysilane while minimizing the self-condensation of the trialkoxysilane. Silica particles treated in this manner show excellent compatibility with organic solvents, monomers and polymers, and provide a more effective usage of the silane surface treating materials.
2. Description of the Prior Art
Numerous methods of treating silica particles to make them more organophilic are known in the prior art. The prior art teachings which involve the reaction of silica and a trialkoxysilane typically involve either the pre-hydrolysis of the trialkoxysilane by mixing the trialkoxysilane with water and optionally acid followed by addition to a silica aquasol, or they involve the reaction of the silica aquasol and the trialkoxysilane in the presence of a major amount of water. For example, International Patent Application WO No. 82/02403 teaches the treatment of silica with a trimethoxysilane by the pre-condensation of the trimethoxysilane in the presence of water and dilute acid prior to admixing with the silica. U.S. Pat. No. 4,188,451 teaches the generation of surface treated silicas by the addition of the corresponding trialkoxysilane to aqueous dispersions of colloidal silica allowing only minor amounts of water-miscible polar solvents. U.S. Pat. No. 4,177,315 teaches the addition of alkyl trimethoxysilanes to an acidified aqueous dispersion of colloidal silica. U.S. Pat. No. 4,027,073 teaches the preparation of surface treated silicas by the addition of trialkoxysilanes to colloidal silica hydrosols.
These prior art methods have several limitations. The self condensation of the trialkoxysilanes results in less efficient use of the silane surface treating agents since much of the silane merely reacts with other silane materials rather than with the silica surface. In addition to requiring the use of high levels of costly materials this extensive self-condensation can result in less than optimum stability in organic solvents. Furthermore, the use of a major amount of water as the solvent for the reaction raises additional difficulties. The excess water typically must be removed prior to mixing the organophilic treated silica with organic materials. Additional time and expense is involved in removing the excess water. Furthermore, removing the water may cause the silica particles, even after they are surface treated, to agglomerate since the sol converts from an aquasol to an organosol in the process.
Additionally, processes which promote self-condensation of the silane, or which use higher levels of silane create an extensive network of the surface treatment on the particles. This can lead to high intrinsic viscosities of the sols which makes them less suitable for applications such as high solid inks or coatings which have very little solvent but still require low application viscosities.
The process taught in this invention overcomes these limits of the prior art by surface treating a silica which is already dispersed in an organic solvent as a silica organosol by utilizing a minor amount of water with respect to the water-miscible organic solvent and by gradually adding the silane to the admixture of water, a catalytic amount of acid and the silica organosol thereby minimizing the self condensation of the silane.
In the prior art methods the untreated silica particles as well as the particles when treated with the self-condensation product of the silane are particularly stabilized by the water solvent. When the water is removed, however, this stability is lost and the particles may agglomerate. The method of the present invention uses a water-miscible organic solvent and thereby limits the amount of water present during the reaction of the silica and the silane.
Although it is not our intent to be bound by theory, it is believed that the low level of water involved in this improved process is not in a sufficient amount to act as a solvent producing an aquasol. Since the sol is not converted to or from an aquasol in this improved process more stable organosols are produced.
The process of the present invention provides a more narrow particle size distribution of the treated particles, minimizes the agglomeration of the particles, involves the removal of less water from the reaction mixture, makes more efficient use of the silane treating agents and provides silica particles with improved stability in organic materials.